1. Field of the Invention
This invention relates generally to improvements in methods and apparatus for producing hot mix asphalt and, more particularly, but not by way of limitation, to an improved method and apparatus for producing hot mix asphalt utilizing new aggregate and a previously manufactured and laid asphalt mix which has been reduced to form recyclable aggregate wherein the new aggregate is employed to protect the mixing apparatus and the recyclable aggregate from excessive temperatures.
2. Description of the Prior Art
As was pointed out in the description of the prior art presented in U.S. Patent applications Ser. Nos. 775,388 and 828,498, various methods and apparatus for utilizing previously manufactured and laid asphalt mix which has been reduced to form recyclable aggregate are well known in the art. However, the various methods and apparatus proposed heretofore have been generally unsatisfactory for one or more of the following reasons:
1. Cleanliness. With the growing awareness of the fragility of our environment, increasingly stringent limitations have been placed on the amount of air-borne pollutants introduced into the atmosphere through the operation of asphalt producing devices. This problem is particularly acute in asphalt recycling devices due to the tendency of the hydrocarbon portion of the recyclable aggregate to decompose at elevated temperatures thereby producing excessive amounts of smoke.
2. Productivity. Rapidly rising labor and machinery costs virtually demand that each piece of paving machinery and crews therefor be kept busy at useful tasks as much as possible. However, the low rates of production generally characteristic of currently proposed recycling methods and apparatus is basically incompatible with this commercial reality.
3. Versatility. In order to be commercially feasible, an asphalt plant capable of utilizing recycled material must also be capable of producing hot mix asphalt using only conventional, new aggregate without a reduction in capacity. Due to the temperature sensitivity of recyclable asphalt aggregate, it has been generally proposed to limit the exposure of the recyclable material to the heating medium through various mechanical devices. However, such devices commonly restrict the operation of the mixing device when employed in conventional hot mix operations. The resultant lack of versatility severely restricts the scope of applicability of such methods and apparatus.
4. Efficiency. Besides making most effective use of the hardware provided, efficiency implies minimum consumption of energy resources in the production of a given quantity of hot mix asphalt. For various reasons, the presently proposed methods and apparatus generally require expensive, additional hardware or consume excessive amounts of energy in efforts to limit the exposure of the recyclable asphalt aggregate to excessive temperatures.